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Care guide · Freshwater · fish

Red-tailed Rasbora

Rasbora borapetensis

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The Red-tailed Rasbora (Rasbora borapetensis) is an active yet peaceful shoaling fish from Southeast Asia. It is also sold as the blackline or black-line rasbora. The species is commonly encountered in the aquarium trade and can suit a community tank when given enough swimming room and a proper shoal. Red colour is confined to the base of the caudal fin; the dorsal fin should be colourless. This helps distinguish it from the very similar R. rubrodorsalis. Gentle water movement, a stable temperature and varied food support good care. Observe the shoal after feeding for even activity.

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At a glance

Temperature
22–26 °C
pH
6.0–7.5
GH
2–12 °dGH
Aquarium length
At least 60 cm
Minimum aquarium volume
At least 68 L
Adult size
At least 5 cm
Group size
At least 8–10 individuals
Temperament
Peaceful

About this species

Seriously Fish gives an adult standard length of about 5 cm. FishBase lists 10 cm total length; because the measurements and source context differ, this is not treated as an equivalent SL figure. The fish has a dark lateral stripe and red at the caudal-fin base. R. rubrodorsalis is distinguished by a red blotch in the dorsal fin. Identify fish by visible features rather than relying on a trade label alone.

Natural habitat

The species occurs in the Mekong, Chao Phraya and Mae Klong drainages, with records from surrounding parts of Southeast Asia. It often inhabits shallow margins of ponds, ditches, reservoirs and canals with still or gently flowing water. It tolerates varied conditions and is sometimes found in turbid sites. Plants combined with open swimming space suit its usual aquarium behaviour.

Aquarium and layout

For a small group, Seriously Fish recommends a base of about 60 × 38 cm and roughly 68 litres. Leave open swimming space and use plants, floating cover or wood for shelter. The species does not need strong flow and may struggle in fast water. A mature filter and regular maintenance remain important. Consider the stated volume together with footprint and group size; it is not a guarantee that every 68-litre tank is suitable.

Water conditions

The species profile gives 22–26°C, pH 6.0–7.5 and GH 2–12 dGH. FishBase supports the temperature and a narrower pH interval but does not provide a complete hardness range. Use the specialist care band as a guide and keep conditions stable. Do not change water chemistry abruptly. Prevent ammonia and nitrite, feed appropriately and carry out regular water changes.

Behaviour and group size

Rasbora borapetensis is very peaceful and often schools closely. Keep at least 8–10 fish so they can show social behaviour and feel less exposed. It can live with other calm fish, but assess tank mates by adult size and feeding competition. Hengel’s rasbora may suit under conditions because of its peaceful nature and overlapping water ranges. Large predators or much faster fish are unsuitable.

Feeding

In the wild this micropredator eats small insects, worms, crustaceans and zooplankton. Provide a varied diet of fine live or frozen foods such as Daphnia and Artemia, supplemented with quality dried food. Flakes or granules must fit the mouth. The specialist source advises against dried food as the sole diet. Offer small portions and check that less assertive fish also eat; remove leftovers to protect water quality.

Breeding

The Red-tailed Rasbora scatters eggs and provides no parental care. Fry may occasionally appear in a mature, densely planted aquarium, but adults eat eggs they find. For deliberate breeding, a separate setup with fine material to protect the eggs is more suitable. Newly hatched larvae first need microscopic food. Results depend on condition and setup; the species profile is guidance, not a promise of a fixed number of fry.

Sources & review

This care guide draws on several specialist sources and was last checked on .

View the sources